The current economics: Installing one of the roofing systems probably adds about $20,000 to $25,000 to the builder's direct costs, Kelly said. The state of California then gives builders a $4,000 to $6,000 rebate after installation, knocking the net additional cost down to $15,000 to $20,000.

Homeowners then get a $2,000 federal tax credit (which goes directly to the homeowner and not to the builder or developer), bringing the price down to $13,000 to $18,000.

Utility savings can range from $500 to $1,300. Payoff, thus, can be as short as 10 years or as long as 36 years. But math class hasn't ended yet. Utility prices continue to climb, so the payoff time will likely be shorter for many buyers because of the energy bill savings, Kelly said. The solar system also adds to the resale value of the house, he said.